Offset printing machines are well known in the art and include a master or plate cylinder having a plate for carrying a transferable image thereon and an impression cylinder for carrying paper to receive the image. A blanket cylinder is interposed between the master cylinder and the impression cylinder such that the blanket cylinder moves eccentrically about a first pivot point to make pressure contact with the master cylinder for receiving the image and subsequently moves eccentrically about a second pivot point for making pressure contact with the impression cylinder to transfer the image to the paper. Such system is disclosed in U.S. Pat. No. 4,691,631.
Two-color offset printing machines are also known and they include a single impression cylinder, two master cylinders and two blanket cylinders. A first blanket cylinder is caused to move eccentrically about a first axis to make contact with a first master cylinder and receive the image therefrom and then move eccentrically about a second pivot point to make contact with the impression cylinder and transfer the first color to the paper on the impression cylinder. The impression cylinder then rotates and carries the image on the paper to the other pair of master and blanket cylinders where the second blanket cylinder moves eccentrically into contact with the second master cylinder to receive the second color and subsequently moves eccentrically into contact with the impression cylinder to transfer the second color to the paper. This system is also disclosed in U.S. Pat. No. 4,691,631.
In both the single and double color offset printing machines, a predetermined contact pressure must be maintained between the blanket cylinder and the master cylinder and between the blanket cylinder and the impression cylinder. Thus, the mounts for eccentrically supporting the blanket cylinder are rotationally forced against a fixed but adjustable stop which is manually adjusted in the prior art to allow a desired amount of pressure at the contact points between the master and the blanket cylinders and between the blanket and impression cylinders, respectively. Adjustment is time-consuming, but readjustment is required for wear and the like as the press is generally set up to print with a relatively fixed thickness of plates, blankets and paper stock. Adjustment, if it is required, is accomplished by varying the thickness of the packing sheets underneath the plate on the master cylinder or underneath the blanket on the blanket cylinder or by varying the center distance of the cylinders.
Small sheet-fed presses, in particular, are required to accommodate a wide range of plate and paper stock thicknesses and thus such semipermanent adjustments mentioned previously are unusable. If the settings are to be made frequently, then they must be done simply and quickly which is difficult as the adjustment of the actuating means requires that the stops on each end of the cylinders must also be accurately readjusted. Because it is necessary for the eccentrics to rotate freely, thereby necessitating some clearance or springiness as in the case of bearings, if rigid stops, such as used in adjusting a web-type press, are not used to rotationally position the eccentrics, there will be a looseness which will allow the printing cylinders to bounce slightly which will be visible in the printing.
Further, the use of one piston to cause the impression/blanket throw-off and another cylinder to cause the plate/blanket throw-off requires a great number of parts that must all work independently but closely together to provide the desired results. Thus, there is first an "off/off" position where the blanket cylinder contacts neither the plate cylinder nor the impression cylinder. Then there is the "off/on" condition where this is contact between the blanket cylinder and the plate cylinder but not between the blanket cylinder and the impression cylinder. Finally there is the "on/off" condition where there is contact between the blanket cylinder and the impression cylinder but not between the blanket cylinder and the plate cylinder.
The present invention discloses a simplified version of an offset printing apparatus in which there is no need for a separate blanket throw-off, the "on/off" condition where there is contact between the blanket cylinder and impression cylinder but not between the blanket cylinder and the plate cylinder. The present invention uses a single eccentric shaft, including an adjustable eccentric tube rotated by a single piston that causes the blanket cylinder rotatably mounted on the eccentric shaft to move first from an "off/off" position to an "off/on" position where the blanket cylinder is in contact with the plate cylinder only and then to an "on/on" position where the blanket cylinder contacts both the plate cylinder and the impression cylinder. This apparatus has the advantage of providing an "inking" position wherein the blanket and plate cylinders make normal contact but there is a minimum gap of 0.010-inch over the thickest paper on the impression cylinder. This is accomplished by associating a solenoid latch with the air cylinder lever to stop it in an intermediate location called the "inking" position. With the inking forms removed from the plate or master cylinder, "printing off" of ink on the blanket cylinder can be accomplished in the "on/on" position.
A double offset printing machine can be constructed with the present invention thus utilizing only one piston for each set of cylinders to cause the blanket cylinder to contact first the plate cylinder and then both the plate and impression cylinders. This present design has only one-third as many parts as the existing mechanisms.
Thus, it is an object of the present invention to provide an offset printing machine that utilizes only one piston to cause the blanket cylinder axis to move along an arc from a first position that separates the blanket cylinder from both the master cylinder and the impression cylinder to a second position that engages the blanket cylinder with the master cylinder for inking and on to a third position that engages the blanket cylinder with both the master cylinder and the impression cylinder for transferring the image on the master cylinder to the paper on the impression cylinder. An eccentric mechanism is used to adjust the eccentric shaft such that the distance from the blanket to the impression cylinder can be adjusted from a minimum to a maximum to compensate for change in blanket thickness, wear or paper thickness.
It is also an object of the present invention to mount the blanket cylinder on an eccentric tube for rotation about the eccentric tube with the eccentric tube adjustably mounted on the eccentric shaft so that by moving the eccentric tube with respect to the shaft, the blanket cylinder/master cylinder distance can also be adjusted for wear and blanket thickness.
It is still another object of the present invention to provide a latch for holding the eccentric shaft in one position with respect to the master cylinder for inking of the blanket cylinder.
It is yet another object of the present invention to provide a two-color offset printer having a single impression cylinder and two master cylinders and two blanket cylinders where each blanket cylinder is operated by a single piston.